Information pertinent to (1) uterine decidualization, (2) trophoblast giant cell differentiation and function, (3) trophoblast -epithelial interactions, and (4) the hormonal control of implantation may be forthcoming through the use of the AY-homozygous implantation failure model system. Upon breeding two heterozygous at the agouti locus (AY/a X AY/a), approximately 25% of the embryos will be homozygous AY/AY and lethal at implantation. The phenotypic expression of the mutant gene, AY appears to be restricted exclusively to trophoblast giant cells; the precise stage at which AY-homozygotes undergo developmental arrest is determined by the degree of differentiation and implantation of giant cells (Eaton and Green, 1963). One can extend the phenocritical period of AY/AY embryos allowing them to temporarily "escape" the lethality either by exogenous progesterone treatments (Eaton, 1968) or by changing the maternal genotype from yellow (AY/a) to non-agouti (a/a) (Robertson, 1942). The AY gene then acts as an elegant experimental tool with which to probe hormonal- morphological correlates during mouse implantation. The objectives of the proposed research are to (1) characterize with light and electron microscopy the sequential morphological events of implantation occurring from 90-120 hr p.c. in decidual tissue, uterine epithelium, unattached blastocysts, and trophoblast-endometrial interactions in the yellow mouse (C57BL/6J-AY/a) genetic system by preparing one uterine horn of a pair for light microscopic quantitation of decidualization, of trophoblast cell attachment-penetration, of blastocyst cell viability, of embryonic developmental stage versus age while preparing the other uterine horn for electron microscopy, (2) characterize lethal yellow trophoblast cells in vitro by their contact relationships, motility, and interaction with endometrial strips using time-lapse cinemicrography and electron microscopy, and (3) gain a more thorough understanding of the hormonal control of mouse implantation.